Ok so I thoroughly tested it both with your latest patch and a completely vanilla rom on regular in game enemies, and it seems the slow effect from the Sonic Arrow has never actually worked.

Edit: so for the tests I gave the Kilwalla (an enemy naturally susceptible to all status effects)16000 hp then engaged the group in prehishory, I confused one and shot a second with the sonic arrow repeatedly. I did this both with your patch and otherwise complete vanilla, they all continued to attack at the same frequency.